1. Field
The present invention relates generally to data communication, and more specifically to an adaptive pilot filter for use in a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of systems, including increased system capacity. A CDMA system is typically designed to implement one or more standards, such as IS-95, cdma2000, IS-856, and W-CDMA standards, all of which are known in the art.
In a wireless communication system, a pilot is often transmitted from a transmitter unit (e.g., a base station) to a receiver unit (e.g., a terminal) to assist the receiver unit perform a number of functions. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., covered with a particular channelization code and spread with a known scrambling code or pseudo-noise (PN) sequence). The pilot may be used at the receiver unit for synchronization with the timing and frequency of the transmitter unit, estimation of the quality of the communication channel, coherent demodulation of a data transmission, and possibly other functions such as determination of the specific transmitter unit having the best link to the receiver unit and the highest data rate supportable by this transmitter unit.
At the receiver unit, a rake receiver is often used to recover the transmitted pilot, signaling, and traffic data. A transmitted signal may be received via multiple signal paths (or multipaths), and each received multipath of sufficient strength may be assigned to and processed by a respective finger processor of the rake receiver. Each finger processor processes the assigned multipath in a manner complementary to that performed at the transmitter unit to recover the data and pilot received via this multipath. The recovered pilot has an amplitude and phase determined by, and indicative of, the channel response for this multipath. The pilot is typically used for coherent demodulation of various traffics transmitted along with the pilot, which are similarly distorted by the channel response. The pilots for a number of multipaths are also used to combine demodulated symbols derived from these multipaths to obtain combined symbols having improved quality.
The quality of the recovered pilot directly impacts the performance of the demodulation process, which may in turn impact the performance of the communication system. A transmitted pilot is typically degraded by channel noise and further distorted by fading in the communication channel. These various phenomena (or channel conditions) combine to make it challenging to estimate the time-varying response (i.e., amplitude and phase) of the communication channel at the receiver unit based on the received pilot.
There is therefore a need in the art for techniques to provide an improved estimate of the time-varying response of a communication channel from a received pilot in a wireless communication system.